The invention relates to heat treatment processes for nickel-chromium molybdenum-alloys having a chromium content of from 12 to 23.5 weight percent.
It is well-known that chromium imparts corrosion resistance to nickel base alloys. Therefore, Nixe2x80x94Crxe2x80x94Mo alloys and particularly those with chromium content of 15 to 24% have been popular for use in corrosive environments such as encountered in the chemical and petrochemical industries.
Age-hardening is a process used in the metallurgical industry to give an alloy composition higher strength, as measured by its yield strength, tensile strength, and by notched stress rupture tests typically used in the art. Various applications demand a combination of high tensile strength and low thermal expansion properties. One such application is in the aerospace industry. Another application is seal rings used in land-based gas turbines. A combination of high tensile strength and ductility is also very useful for bolts. Because of the demanding operating conditions and performance parameters for metal products in these applications, various methods of age-hardening have been used. One common technique is to heat the alloy to a selected high temperature, hold the alloy at that temperature for a period of time and then cool the alloy to room temperature. For some alloy compositions, the alloy may be heated to one temperature, cooled, heated again to a second temperature and cooled. Examples of these processes are disclosed in U.S. Pat. No. 3,871,928. The temperatures and time periods used to age harden an alloy depend upon the composition of the alloy. For all age-hardenable commercial alloys there are established times and temperatures used that have become standard in the industry because they are known to produce the desired properties. For Nixe2x80x94Crxe2x80x94Mo alloys having high chromium content, that is chromium greater than 12%, the general view in the art is that heat treatment beyond the initial annealing in an effort to improve mechanical properties would be impractical due to the lengthy required times (hundreds to thousands of hours) and such treatments simply have not been done.
Solid-solution strengthened nickel-chromium-molybdenum (Nixe2x80x94Crxe2x80x94Mo) alloys and nickel-molybdenum (Nixe2x80x94Mo) alloys are widely utilized for commercial applications in the chemical industry, for example. Generally, considered to be single phase materials, discounting the presence of minor carbide phases, alloys such as these are not usually considered responsive to heat treatment, and are therefore used in the annealed condition. There are exceptions, where some particular alloys do exhibit a commercially exploitable age hardening response. However, in these instances the age-hardening response observed is attributable to other elements, such as niobium, aluminum and titanium being present in the alloy composition. The exception to this is HAYNES(copyright) 242(trademark) alloy which will be discussed later. The fact that Nixe2x80x94Crxe2x80x94Mo and Nixe2x80x94Mo alloys are not commercially age-hardenable does not mean that they do not exhibit any metallurgical response to thermal exposure at intermediate temperatures. Actually, alloys of this type can exhibit complex secondary phase reactions when exposed in the temperature range from about 1000xc2x0 F. to 1600xc2x0 F. Unfortunately, the phases which form can often be deleterious to both alloy ductility and other aspects of service performance. This is particularly observed with Nixe2x80x94Mo alloys containing about 25 to 30% molybdenum. In such materials, exposure at temperatures from about 1000xc2x0 F. to 1600xc2x0 F. can result in the rapid formation of embrittling Ni3Mo or Ni4Mo phases in the microstructure. This can be a problem for both component manufacturing and for component performance.
For lower molybdenum, higher chromium, content Nixe2x80x94Crxe2x80x94Mo alloys with about 16% molybdenum and 16% chromium weight percent content, the occurrence of these particular intermetallic phases is not usually observed after short term thermal exposures. With longer term exposure at temperatures from about 1000xc2x0 F. to 1200xc2x0 F., there is a distinctly different metallurgical response. After about 500 to 1000 hours the presence of the phase Ni2(Mo,Cr) is evidenced in the microstructure. A long-range-ordered phase, with structure similar to that of Pt2Mo, the Ni2(Mo,Cr) phase serves to significantly increase the strength of these materials without a severe loss of ductility. The one major drawback is the prolonged aging time required to produce this phase.
There are several United States patents that disclose Nixe2x80x94Crxe2x80x94Mo alloys. U.S. Pat. No. 4,818,486 discloses a low thermal expansion nickel based alloy that contains 5% to 12% chromium and 10% to 30% molybdenum. The patent teaches that the aging times typically required to obtain desired hardness without deleterious phases being formed is well over 1000 hours at temperatures of 1200xc2x0 F. to 1500xc2x0 F. for most Nixe2x80x94Moxe2x80x94Cr alloys. However, the aging time to harden the alloy composition disclosed in the ""486 patent is as little as 24 hours at 1200xc2x0 F. The alloy of this patent has been marketed under the trademarks 242 alloy and HAYNES 242 alloy. HAYNES 242 alloy has been sold for applications requiring high tensile strength and a low coefficient of thermal expansion. Other beneficial properties of the 242 alloy include good thermal stability, good low cycle fatigue resistance, and excellent containment capabilities due to its tensile strength and ductility. HAYNES 242 alloy consists of about 8% (weight percent) chromium, about 20-30% molybdenum, about 0.35% to up to about 0.5% aluminum, up to 0.03% carbon, up to about 0.8% manganese, up to about 0.8% silicon, up to about 2% iron, up to about 1% cobalt, up to about 0.006% boron, and the balance weight percent being nickel.
There is a need for a shorter commercially exploitable age hardening process for Nixe2x80x94Moxe2x80x94Cr alloys with higher Cr levels ( greater than 12% Cr) than found in U.S. Pat. No. 4,818,486 that avoids formation of deleterious Ni3Mo and Ni4Mo phase, as well as mu-phase occurrence.
Another Nixe2x80x94Crxe2x80x94Mo alloy is disclosed in U.S. Pat. No. 5,019,184 to Crum et al. That alloy contains 19% to 23% chromium and 14 to 17% molybdenum. The patent discloses homogenization heat treatment at temperatures ranging from 1149xc2x0 C. to 1260xc2x0 C. for periods of from 5 to 50 hours. The purpose of the treatment is to produce a corrosion resistant alloy having a desired microstructure rather than to strengthen the alloy. No tensile strength data is given for any of these samples disclosed in the patent. The alloy of this patent has been commercialized under the designation INCONEL(copyright) alloy 686.
Yet another corrosion resistant Nixe2x80x94Crxe2x80x94Mo alloy is disclosed in U.S. Pat. No. 4,906,437 to Heubner et al. This alloy contains 22% to 24% chromium and 15% to 16.5% molybdenum. There is no disclosure of any heat treatment or age hardening of this alloy. The alloy disclosed in this patent has been commercialized under the designation VDM NICROFER 923 h Mo or Alloy 59.
A high yield strength Nixe2x80x94Crxe2x80x94Mo alloy is disclosed in U.S. Pat. No. 4,129,464 to Matthews et al. This alloy contains 13% to 18% chromium and 13% to 18% molybdenum. The patent says that the alloy could be aged using a single step aging treatment of at least 50 hours at 900xc2x0 F. to 1100xc2x0 F., but all examples are aged 168 hours or more. The statement that at least 50 hours is required was an extrapolation of the results obtained from a 168 hours aging treatment. The patent reports data for three alloys numbered 1, 2 and 3. Alloy 1 is commercially available under the trademark HASTELLOY C-276 alloy. Alloy 2 is commercially available as HASTELLOY C-4 alloy. Alloy 3 is commercially available as HASTELLOY S alloy.
We provide an improved two-step age hardening process for certain nickel-chromium-molybdenum alloys containing from 12% to 23.5% chromium that results in higher yield strength, high tensile strength and comparable other mechanical properties as those observed with the current age-hardening process used in the art, such properties being measured by yield strength, tensile strength, and tensile ductility tests at room temperature. This process works only for those alloys in which the other alloying elements are present in amounts so that the composition has a P value within the range of 31.2 to 35.9 where P is defined by the equation:
xe2x80x83P=2.46 Al+0.19 Co+0.83 Crxe2x88x920.16 Cu+0.39 Fe+0.59 Mn+1.0 Mo+0.81 Zr+2.15 Si+1.06 V+0.39 W+0.68 Nb+0.52 Hf+0.45 Ta+1.35 Ti
The improved age-hardening process involves an alloy with this P value that has been given an age hardening treatment at about 1300xc2x0 F. for at least 8 hours and preferably for about 12 to 20 hours, furnace cooling to a temperature from about 1000xc2x0 F. to about 1325xc2x0 F. and holding the material at that temperature for at least 8 hours and preferably for about 28 to 36 hours, followed by air cooling to room temperature. A Nixe2x80x94Crxe2x80x94Mo alloy having 12% to 23.5% chromium and treated with this two-step heat treatment or age-hardening process shows improved or comparable tensile strength to the standard aging process used in the lower chromium level 242 alloy. Because of the combination of high yield strength and ductility properties, the alloy and two-step aging process significantly increase affordability of this alloy for applications requiring such properties.
Recently we have observed that a two stage heat treatment performed over a total time of from 10 to 20 hours can provide acceptable mechanical properties. However, when a similar two step process was tried for Nixe2x80x94Crxe2x80x94Mo alloys with higher chromium content, some alloys had acceptable properties while other alloys did not. A higher chromium content is desirable for alloys intended for use as corrosion-resistant alloys in the chemical process industry. Consequently, we saw a need for determining whether there existed a definable class of high chromium content alloys which would exhibit acceptable mechanical properties when subjected to a relatively short two step aging process.